It is customary for shoe manufacturers to reinforce the toe end of the upper to obtain improved wear and retention of shape. It is accepted practice throughout the shoe industry to obtain such reinforcement by the application of a thermoplastic stiffening resin, sometimes referred to as a box-toe resin, to the toe portion of the upper. The thermoplastic resin is applied as a melt and upon cooling forms a stiff resilient reinforcing coating on the upper.
For a thermoplastic resin to be an acceptable stiffener in this application the resin must satisfy the following requirements: first of all, the resin should have some adhesive character; it should have a low melt viscosity, preferably less than 150 poise at 190.degree. C.; the resin should set rapidly to prevent "welding" of stacked assemblages of the manufactured articles; and the resin must be stiff to impart and retain the desired shape but it must also have sufficient flexibility, even at low temperatures, to resist cracking upon impact and to "snap back" to its original shape. This latter property or more correctly, balance of properties is sometimes referred to as "rigid flexibility" and is perhaps the most troublesome and difficult property to develop in a thermoplastic resin, particularly in polyamide resins. Heretofore it has only been possible to obtain this difficult yet critical balance of properties by blending the thermoplastic polyamide resin with a minor amount of one or more other compositions including other polyamide resins. A single polyamide resin possessing all the aforementioned characteristics so as to be useful in shoe manufacturing operations in neat form has heretofore not been available.
Polyamide resins derived from polymeric fatty acids and conventional diamines are unacceptable since they become brittle upon aging and have a tendency to crack when flexed, especially at low temperatures. The use of mixed diamines, e.g. mixtures of ethylene diamine and hexamethylene diamine, with polymeric fatty acids improves the flexibility at low temperatures, however, the resins lack resiliency and memory and are unable to "snap-back" to their original configuration. The incorporation of a short-chain dibasic acid with the polymeric fatty acid improves the stiffness but reduces the impact resistance of the resin to a point where it is unacceptable.
U.S. Pat. No. 3,499,853 discloses thermoplastic adhesives derived from relatively low molecular weight ether diamines, by themselves or in combination with ethylene diamine, and polymeric fatty acids. The resulting thermoplastic copolyamides have high resistance to peel and excellent adhesive properties. When mixtures of ethylene diamine and ether diamines are employed the equivalent ratio of ethylene diamine to ether diamine can range as high as about 0.9:0.1 but, more preferably, will be between about 0.7:0.3 and about 0.6:0.4. Even though the copolyamides of U.S. Pat. No. 3,499,853 have excellent impact resistance they are not suitable box-toe resins since they have unacceptable resilience and are too soft. British Patent No. 1,319,807 also discloses the use of copolyamide resins derived from polymeric fatty acids and low molecular weight aliphatic ether diamines.